Bifidobacteria and lactic acid bacteria are used as probiotics for various kinds of foods. For example, bifidobacteria and lactic acid bacteria are prepared in forms of powders, capsules, tablets, and the like, for the application to a wide range of food products including health foods, confectioneries, and baby milk powder. Moreover, applications of bifidobacteria and lactic acid bacteria as probiotics to the fields of medical services and livestock foods have also started.
Probiotics confer a beneficial effect on the human and animal health by proliferating themselves inside the intestine when administered into the host human or animal. Therefore, it is important that probiotics are kept alive. Although it is very difficult to mix a bifidobacterium or a lactic acid bacterium in a product while maintaining its survival cell count within the guarantee period (best-before period), some techniques have been developed in the past, and now, many probiotic-containing foods, pharmaceuticals, and feeds (hereunder, referred to as probiotic products) are on the market.
For the development of probiotic products, a great number of items have to be examined, for example, it is necessary to predict the change in the survival cell count in the product, to set the best-before period of the product, and to check the efficacy of the product. In addition, it is also necessary to clarify the guaranteed cell count in the probiotic product within the guarantee period, in terms of the quality assurance of the product for the consumer.
For this reason, during the development of probiotic products, it is necessary to measure the survival cell count of a probiotic strain in the product within the storage period by performing an actual storage test so as to evaluate the probiotic survivability. However, as the guarantee period of probiotic products is often set as very long, as much as one to three years, it is necessary, for measuring the survival cell count of a probiotic strain at the completion of the guarantee period and for setting the guaranteed cell count, to carry out a long term storage test to be comparable to the guarantee period. Thus, it takes time to supply probiotic products in the market.
Therefore, in order to shorten the storage test of probiotic products, techniques for performing an accelerated test at a high storage temperature to thereby estimate the survival cell count using the result of the accelerated test, are being studied (refer to Non-patent Documents 1 to 4).
However, since the storage temperature has to be set high so as to shorten the span of the accelerated test and such a high temperature leads to an increase in the bacterial inactivation rate, it is very difficult for this test to accurately estimate the survivability at a normal storage temperature. Moreover, if the set temperature of the accelerated test is slightly higher than normal temperature, it is not possible to sufficiently shorten the span of the storage test.
Incidentally, it is known that the survivability of bifidobacteria changes depending on the moisture content and the water activity value of the product (refer to Non-patent Document 5). Furthermore, it is known that the survivability of bifidobacteria changes depending not only on the water activity value of the product but also on the storage temperature of the product (refer to Non-patent Document 6). In addition, it is known that bifidobacteria have different survivabilities per each strain (Non-patent Document 7). Moreover, reportedly, the survivability of lactic acid bacteria also changes depending on the storage temperature and the water activity value (refer to Non-patent Document 8).    [Non-patent Document 1]: Damjanovic, V. and Radulovic, D.), “Cryobiology”, (UK), 1968, Vol. 5, p. 101-104    [Non-patent Document 2] Achour et al., “Journal of Chemical Technology and Biotechnology”, (UK), 2001, Vol. 76, p. 624-628    [Non-patent Document 3] Ziadi et al., “Biochemical Engineering Journal”, (Holland), 2005, Vol. 24, p. 141-145    [Non-patent Document 4] Portner et al. “Cryobiology”, (UK), 2007, Vol. 54, p. 265-270    [Non-patent Document 5] Nagawa et al., “Journal of Dairy Science”, (USA), 1988, Vol. 71, p. 1777-1782    [Non-patent Document 6] Tatematsu et al. “Japanese Journal of Freezing and Drying”, 1982, Vol. 28, p. 40-45    [Non-patent Document 7] Simpson et al., “Journal of Applied Microbiology”, (UK), 2005, Vol. 99, p. 493-501    [Non-patent Document 8] Higl et al., “Biotechnology Progress”, (US), 2007, Vol. 23, p. 794-800